Test structures are fabricated in order to enhance defect detection and/or analysis of a microfabrication manufacturing process. Test structures may be included in a variety of objects, such as but not limited to integrated circuits, masks (for fabricating integrated circuits, flat panel displays and the like), MEMS devices and the like. They may be located at various locations on these objects, such as in the integrated circuit die or in scribe lines on semiconductor wafers. In many cases the size of a defect is much smaller than the size of the test structure and there is a need to locate the defect within the test structure in order to perform classification and root cause analysis. The localization of the defect is difficult and time consuming, especially in the context of integrated circuit manufacturing, and failure analysis devices, such as Defect Review Scanning Electron Microscope (DR-SEM) that are utilized during said manufacturing process.
Usually, test structures include one, two or more electrical conductors that may be shaped in various manners, such as a comb, serpentine, nest, via chain and the like that are known in the art. A defective test structure may be characterized by hard defects (electrical short or electrical open, i.e. isolated) and soft defects (high resistance vias or shorts resulting from metal threads or stringers).
PCB electro-optic high speed sampling-based probing of voltages on PCBs is known as described for example in US patents to Paul Meyreueix et al of Schlumberger U.S. Pat. No. 5,272,434 and U.S. Pat. No. 5,394,098. An electro-optic (EO) coating is applied to a PCB and is then used to sense and sample the voltage waveform as a function of time on PCB conductors under the EO coating.
The Microloop Product by KLA-Tencor of San Jose Calif. uses a combination of the KLA-Tencor eS20 e-beam inspection system and KLA-Tencor eV300 DR SEM for non-contact inspection, defect localization and classification of defects in electrical test structures. Microloop cannot detect or localize soft defects as the beam induced current (typically <100 nA and often <1-10 nA) through a high resistance defect often does not result in a sufficient voltage difference across the defect for that voltage difference to be detected in a voltage contrast image. Microloop is also prohibitively expensive for some manufacturers to employ routinely due to the cost and complexity of the vacuum and loadlock systems required by the DR and EBI SEMs.
Alternately a voltage contrast e-beam prober, for example, an IDS 10000 e-beam prober from NPTest of San Jose, Calif. (formerly Schlumberger Semiconductor Solutions) with mechanical probes or a probecard in the vacuum chamber, can, using direct electrical connections (with mechanical probes or probecard), inject larger currents (than are possible with e-beam) into test structures. The larger current when passing through a soft electrical defect produces a larger voltage and therefore a larger voltage contrast signal that can be more readily detected. However, mechanical probes are difficult to manipulate accurately or reliably inside a vacuum chamber and generate micro-particle contamination that is unacceptable in cleanroom tools, particularly in-line SEMs (DR, EBI & CD) and generally in microfabrication manufacturing.
OBIRCH—Optical Beam-Induced Resistance CHange—Nikawa-san of NEC publications at ISTFA and IRPS 1999 and 2000: an optical beam is raster scanned over the structure and the supply current or voltage is monitored. The optical beam (usually IR) heats the structure locally, and temporarily increasing the resistance of the element heated. When a defective high resistance structure is heated, the resistance change is often greater and results in a larger and readily detectable change of supply current (or voltage or in the case of a constant current supply). An OBRICH image can be produced by plotting the change in supply current or voltage against the position of the optical beam in the raster—defects show as bright (or dark) areas in the image corresponding with the larger change in current or voltage induced by the presence of the defect.